Check valves are a necessary component of high pressure positive displacement pumps, in particular pumps commonly known as plunger pumps, piston pumps and hydraulic exchange pumps, all of which have one or more chambers which alternately fill and discharge in a cyclical sequence. These pumps generally have 2-6 individual pumping units the cycles of which are staggered to more nearly approximate an even flow. The number of strokes or cycles per minute for each valve generally ranges from 2-15 for hydraulic exchange pumps and 30-300 for piston pumps and plunger pumps.
As fluid, pressurized by a piston, plunger or a pressure vessel enters the check valve, its flow rate rapidly increases to a maximum and then falls to zero and remains there until completion of the cycle. While the flow rate of plunger pumps and piston pumps when plotted against time generally follows a sinusoidal curve, the flow rate of a hydraulic exchange pump remains nearly constant at the maximum rate over much of the discharge portion of the pumping cycle. While water hammer and associated problems occur to some extent with piston pumps and plunger pumps, the phenomenon may be more severe with hydraulic exchange pumps because of the very rapid falloff in flow at the end of the discharge portion of the cycles, which results in seating of the ball with extreme inertia. The balls and seats tend to deteriorate nearly as rapidly as those of piston pumps for example, in spite of the slower speed (longer cycle time). In some applications, the noise generated by such a large high pressure pump may be a problem.
These types of pumps are typically used to pump slurries which are very erosive, for example, coal slurries, ore suspensions, or sludges containing sand and grit, at pressures of 50 to 3500 psig. Such materials may damage the ball and valve seat as the ball rapidly drops from the maximum flow position of the valve seat. This effect is magnified when the ball becomes seated in the same position on each stroke or cycle, i.e. without turning to produce wear evenly over its entire surface.
When the flow of fluid to a check valve is quickly stopped, as in these types of pumps, the ball does not seat immediately due to its inertia. The resulting blackflow or slip of the fluid tends to reduce the net pumping rate and pump efficiency.
The prior art is replete with check valves for many purposes, and many such valves provide an element for engaging the valve element of a check valve and controlling the movement of the valve element as it moves away from the valve seat and then against a valve lift limiter under the pressure of the fluid being handled by the valve. Examples of such patents are U.S. Pat. Nos. 1,877,938; 2,097,629; 2,481,713; 2,964,310; and 3,559,678.
It is also known to provide a cage around a valve in such a valve element for closely guiding the valve element in its movement from the position where it is seated on the valve seat to the position where it engages the valve lift limiter, see for example U.S. Pat. Nos. 1,642,724 and 2,081,462.
However, these valves do not overcome the above described problems. In all of these valves, especially those where the element actually engages the valve element at all times, the tendency is for the valve element to seat on the valve seat with the same part of the valve element contacting the valve seat each time the valve element seats. While this does not cause any great difficulty in valves which do not encounter very severe conditions during their use, it does cause undue wear on the valve elements of valves which are used under very severe conditions such as described above.
It would be desirable for the art to have a check valve in which the valve element is free to move each time it is shifted away from the valve seat so that wear is distributed over the whole valve element, yet in which the movement of the valve element is controlled during most of its movement away from the valve seat. It would further be desirable for the art to have such a valve which could be provided by making a simple change in an existing valve structure.